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Idaho basin outlook report PDF

26 Pages·1993·2.3 MB·English
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Historic, Archive Document Do not assume content reflects current scientific knowledge, policies, or practices. \es aTD22^ Idaho t of .X2I33 Soil Conservation Service Basin Outlook Report A January 1 Basin Outlook Reports and Federal - State - Private Cooperative Snow Surveys For more watersupply and resource management information, contact: Your local Soil Conservation Service Office or Soil Conservation Service Snow Surveys 3244 Elder Street, Room 124 Boise ID 83705-4711 (208) 334-1614 How forecasts are made Most of the annual streamflow in the Western United States originates as snowfall that has accumulated high in the mountains during winter and early spring. As the snowpack accumulates, hydrologists estimate the runoff that will occur when it melts. Predictions are based on careful measurements of snow water equivalent at selected index points. Precipitation, temperature, soil moisture and antecedent streamflow data are combined with snowpack data to prepare runoff forecasts. Streamflow forecasts are coordinated by Soil Conservation Service and National Weather Service hydrologists. This report presents a comprehensive picture of water supply conditions for areas dependent upon surface runoff. It includes selected streamflow forecasts, summarized snowpack and precipitation data, reservoir storage data, and narratives describing current conditions. Snowpack data are obtained by using a combination of manual and automated SNOTEL measurement methods. Manual readings of snow depth and water equivalent are taken at locations called snow courses on a monthly or semi-monthly schedule during the winter. In addition, snow water equivalent, precipitation and temperature are monitored on a dally basis and transmitted via meteor burst telemetry to central data collection facilities. Both monthly and daily data are used to project snowmelt runoff. Forecast uncertainty originates from two sources: uncertainty of future hydrologic and (1) climatic conditions, and (2) error in the forecasting procedure. To express the uncertainty in the most probable forecast, four additional forecasts are provided. The actual streamflow can be expected to exceed the most probable forecast 50% of the time. Similarly, the actual streamflow volume can be expected to exceed the 90% forecast volume 90% of the time. The same is true for the 70%, 30%, and 10% forecasts. Generally, the 90% and 70% forecasts reflect drier than normal hydrologic and climatic conditions; the 30% and 10% forecasts reflect wetter than normal conditions. As the forecast season progresses, a greater portion of the future hydrologic and climatic uncertainty will become known and the additional forecasts will move closer to the most probable forecast. All programs and services of the USDA Soil Conservation Service are offered on a nondiscriminatory basis, without regard to race, color, national origin, religion, sex, age, marital status, or handicap. IDAHO WATER SUPPLY OUTLOOK REPORT JANUARY 1993 1, SUMMARY After almost six years of drought conditions, Idaho’s mountain snowpack is off to a promising start for the 1993 water year. Snowpacks throughout the state are near or above normal for January 1. Soil moisture and ground water conditions are very dry, however, and could have an adverse effect on runoff. Reservoir storage will play a major role in the water supply for 1993. Many reservoirs across southern Idaho are nearly empty, and water users will be almost entirely dependent upon snowpack and the resulting runoff for their water supply next summer. Over half of the winter accumulation season still lies ahead, and the next three months will determine the fate of Idaho’s 1993 water supply. SNOWPACK Snowfall started around the first of November in Idaho, and frequent storms increased the mountain snowpacks during the early winter months. Powder snow and excellent coverage have delighted skiers throughout the state. Lower elevation srowpack throughout most of the state is well above average. In a reversal of previous years’ trends, southern Idaho snowpacks are among the best in the state. Snowpacks currently range from 130 to 155% of average across the southern edge of the state, with the exception of the Bear River area which reports near average conditions. The northern two-thirds of the state reports 85 to 120% of average snowpacks. The upper Snake basin in Wyoming reports 96% of average snowpack. By January 1, approximately 40% of the season’s snowpack is on the ground. With over half of the — winter season still ahead, there is plenty of time for snowpack conditions to change for better or worse! PRECIPITATION The water year began with very little mountain rainfall prior to the beginning of snow accumulation. As a result, most soils are very dry under the mountain snowpack and could absorb significant snowmelt, reducing runoff yield. Overall, October was dry in the Panhandle and wet in the south, especially in the central mountains. This trend reversed in November as mountain precipitation in southern Idaho turned to a drier pattern. During December, precipitation was near or above normal throughout most of the state with well above average precipitation in southwest and southcentral Idaho. Water year to date mountain precipitation is around 90% of average in northern Idaho and the upper Snake River basin in Wyoming, near average in the central mountains and Bear ^ver basin, and 120% of average in the south. Temperatures so far this winter have been below normal. The months of November, December, and January A combined account for nearly half of the average annual mountain precipitation. wet January will almost certainly be needed to ensure adequate water supplies in the summer of 1993. RESERVOIRS Reservoir carryover storage is one of the few known quantities influencing this NOT M^y summer’s water supply... and the news is good. reservoirs in soutHem Idaho are holding record low storage amounts as a result of the sus^n^ drought of the last several years. Northern Idaho reservoirs are in somewhat better sHaj^. Reservoirs of specific concern, holding less than 25 percent of capacity; include t^^^ Boise system, Little Wood, Mackay, Oakley, S^mbn Falls, Owyhee, Be^ t^e; WA Montpelier Creek reservoirs. As usual, storage in the upper Sn^e ^d Payette system is somewhat better; these systems report 32 and 47% of capacity, r^pect^ivel^ Some of the storage in the Payette system will be used to back fill Browhl^ Reservoir, so these figures should drop as water is released. _ ^ ^ _ Note: SCS reports reservoir information in terms of ‘usable’ contents, which incliiides both active and inactive storage. Other operators may report reservoir contents ih different terms. STREAMFLOW October through December streamflow was below to well below normal tbrSughout Idaho, a continuing sign of low soil moisture conditions due to the extended drought. Fall streamflow was 50 to 70 percent of average across central and southern Idaho arid 70 to 80 percent of average in northern Idaho and the upper Snake. As a result, reservoirs are refilling slower than normal and many operators have r^iiced the outflows to bare minimums needed for protection of fisheries., Sfieamflpw for©^& for the coming spring and summer are in the near average range throughout Id^o with the exception of the Owyhee, Bruneau, Oakley and Salmon Falls basins iri southern and southwestern Idaho where abundant snowpack has produced forecasts well above normal for the first time in many years. Panhandle Region January 1993 1, Mountain Snowpack (inches) Mountain Precipitation PANHANDLE REGION PANHANDLE REGION *- CURRENT MONTHLY ->l< AVERAGE WATER YEAR TO DATE 250. MAXIMUM MINIMUM 200. Ol50_ f A V el00. r a g 50. OCT NOV DEC Based on selected SNOTEL sites WATER SUPPLY OUTLOOK The Panhandle region received slightly below normal precipitation during October and November and near normal precipitation during December. Mountain precipitation for the water year stands at 88% of average. Several major storm systems moved through the Panhandle region during December and produced blizzard conditions in the lower elevation areas. Snowpacks are well above normal in the lower elevations, and near average in the higher elevations. Currently, snowpacks range from 138% of average in the lower elevation Palouse basin to 97% of average on the Pend Oreille basin. Streamflow forecasts range from 84% of average for the inflow to Pend Oreille Lake to 108% of average for Coeur d’Alene River. r PANHANDLE REGION Streamflow Forecasts January 1, 1993 «====== Drier ======= Future Conditions ======== Wetter ..... Forecast Point Period 90% 70% 50% (Most Probable) 30% 10% 30-Yr Avg. (1000AF) (1000AF) (1000AF) (% AVG.) (1000AF) (1000AF) (1000AF) KOOTENAI at Leonia (1,2) APR-JUN 3120 4430 5020 88 5610 6920 5701 APR-JUL 3970 5600 6340 88 7080 8710 7199 APR-SEP 4550 6420 7270 88 8120 9990 8275 CLARK FK at Whitehorse Rpds (1,2) APR-JUN 4660 7560 8870 88 10200 13100 10050 APR-JUL 5470 8860 10400 89 11900 15300 11730 APR-SEP 5970 9710 11400 88 13100 16800 12910 PEND OREILLE LAKE inflow (1,2) APR-JUN 4830 8320 9910 87 11500 15000 11390 APR-JUL 6020 9720 11400 87 13100 16800 13150 APR-SEP 6610 10700 12500 87 14300 18400 14370 PRIEST nr Priest River (1,2) APR-JUL 390 605 700 86 795 1010 814 APR-SEP 420 645 750 86 855 1080 868 COEUR D'ALENE at Enaville (1) APR-JUL 395 665 785 102 905 1180 769 APR-SEP 415 695 825 102 955 1240 809 ST. JOE at Calder APR-JUL 780 970 1100 94 1230 1420 1169 APR-SEP 730 1030 1160 94 1290 1580 1237 SPOKANE nr Post Falls (1,2) APR-JUL 785 2010 2570 98 3130 4360 2627 APR-SEP 950 2090 2670 98 3250 4410 2720 PANHANDLE REGION PANHANDLE REGION Reservoir Storage (1000 AF) - End of December Watershed Snowpack Analysis - January 1, 1993 Usable *** Usable Storage *** Nunber This Year as % of Reservoi Capacity This Last Watershed of ===== Year Year Avg Data Sites Last Yr Average HUNGRY HORSE 3451.0 1438.0 2110.0 2586.0 Kootenai ab Bonners Ferry 24 80 73 FLATHEAD LAKE 1791.0 1021.0 1022.0 1305.0 Moyie River 2 78 61 NOXON RAPIDS 335.0 311.2 322.3 317.1 Clark Fork River 44 81 94 PEND OREILLE 1561.3 486.4 541.2 744.9 Priest River 4 111 97 COEUR D'ALENE 238.5 44.5 140.6 130.5 Pend Orei1le River 60 88 97 PRIEST LAKE 119.3 51.8 21.8 54.8 Rathdrum Creek 5 299 129 Hayden Lake 0 0 0 Coeur d'Alene Hiver 5 99 104 St. Joe River 2 97 107 Spokane River 12 129 112 Palouse River 1 200 138 * 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual flow will exceed the volumes in the table. The average is computed for the 1961-1990 base period, (1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels. (2) - The value is natural flow - actual flow may be affected by upstream water management. Clearwater River Basin January 1993 1, Mountain Snowpack (inches) Mountain Precipitation CLEARWATER RIVER BASIN CLEARWATER RIVER BASIN >|<— ->|< CURRENT — - AVERAGE WATER YEAR TO DATE 250 MAXIMUM n t 0 150 f A V 100 e r a g ® 50 0 OCT NOV DEC Based on selected SNOTEL sites WATER SUPPLY OUTLCX)K The water year started with below normal mountain precipitation in October and then improved with near average precipitation falling in November and December. Mountain precipitation for the water year is still slightly below average at 93% of average. Overall, the Clearwater basin is reporting a near normal snowpack at 106% of average. The snowpack in the lower elevations is above normal while the snowpack in the higher elevations is near normal. Dworshak Reservoir storage is 75% of capacity which is normal for this time of year. Streamflow forecasts range from 90 to 110% of average for the Clearwater basin. Current conditions indicate that water supplies should be adequate in the Clearwater basin during 1993. r CLEARWATER RIVER BASIN Streamflow Forecasts - January 1, 1993 «====== Drier ====:== Future Conditions ======= Wetter Forecast Point Forecast | Period 90% 70% 50% (Most Probable) 30% 10% 30-Yr Avg. | 1 | 1 (1000AF) (1000AF) (1000AF) (% AVG.) (1000AF) (100QAF) (1000AF) 1 1 1 DWORSHAK RESERVOIR inflow (1) APR-JUL 1720 2580 2970 110 3360 4220 2700 1 1 APR-SEP 1830 2740 3160 110 3580 4490 2875 1 1 1 1 1 1 CLEARWATER at Orofino (1) APR-JUL 2050 3570 4260 90 4950 6470 4718 1 1 APR-SEP 2170 3770 4500 90 5230 6830 4976 1 I 1 1 1 1 CLEARWATER at Spalding (1,2) APR-JUL 3900 6470 7640 100 8810 11400 7618 1 1 APR-SEP 4110 6830 8070 100 9310 12000 0052 1 1 1 1 CLEARWATER RIVER BASIN CLEARWATER RIVER BASIN Reservoir Storage (1000 AF) - End of December Watershed Snowpack Analysis - January 1, 1993 Usable *** Usable Storage *** Number This Year as % of | Reservoi Capacity This Last Watershed I Year Year Avg Data Sites Last Yr Average I DWORSHAK 3467.8 2627.0 1453.1 2431.0 North Fork Clearwater 11 94 105 | 1 1 Lochsa River 4 89 98 1 1 1 Selway River 5 94 108 1 1 1 Clearwater Basin Total 19 94 106 1 90%, 70%, 30%, and 10% chances of exceeding are the probabilities that the actual flow will exceed the volumes in the table. The average is computed for the 1961-1990 base period. (1) - The values listed under the 10% and 90% Chance of Exceeding are actually 5% and 95% exceedance levels. (2) - The value is natural flow - actual flow may be affected by upstream water management.

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